A lithium secondary battery is characterized by its high energy density and is a battery composed of: a negative electrode of an active material capable of storing/releasing lithium metal or lithium ions; a positive electrode of an active material capable of storing/releasing lithium ions or anions; a nonaqueous electrolyte solution; and the like. Further, such characteristics as high output and longer operating life due to the capability for repeated charging/discharging a considerable number of times, have been attracting attention.
Lithium secondary batteries are already mainly employed as the secondary batteries for use in portable devices, such as compact video cameras, cellular phones and notebook computers, due to their high energy density characteristics. In addition, intensive research is being carried out into applications of such batteries as a power source for energy regeneration in hybrid type electric cars, night storage electric power systems, and the like, due to their characteristics of high output and long operating life. In view of these circumstances, further enhancements of the energy density, output and operating life are still desired.
In order to increase the energy density of lithium secondary batteries, a method of increasing the packing density of active materials in a battery is generally adopted. In such cases, the amount of an electrolyte solution impregnated into pores in the electrodes will be inevitably reduced, and therefore a lowering of the operating life of the battery tends to occur, due to the smaller amount of the electrolyte solution. Furthermore, in applications such as hybrid electric cars and night storage electric power systems, an operating life of several years or longer is required. Based on the above reasons, an electrolyte solution is desired that will further improve the operating life and enhance the charge/discharge performance of a battery.
Towards these aims, the addition of various kinds of additives has been considered as measures for improving the lifespan characteristics of a battery. For example, there is proposed adding a sultone compound having a double bond at a specified site to a nonaqueous electrolyte solution (for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-329528). In this case, however, although there is a large effect for improving the operating life of the battery, there is a fear that the internal resistance of a battery will be increased, lowering the charge/discharge load characteristics thereof. There is also proposed adding fluoroethylene carbonate to a nonaqueous electrolyte solution (for example, JP-A No. 7-240232 and JP-A No. 2004-47131). In this case, however, although there is only a small risk that the internal resistance of the battery will be increased, there is a fear that there will be little improvement in the charge/discharge cycle characteristics at high temperature. There is also a proposal to add vinylene carbonate to a nonaqueous electrolyte solution. In this case, however, the effect of improving the lifespan characteristics of a battery at high temperature will not be satisfactory, even though the possibility of increasing the internal resistance of the battery is small.    Patent document 1: JP-A No. 2002-329528    Patent document 2: JP-A No. 7-240232    Patent document 3: JP-A No. 2004-47131